Radiation-sensitive elements having a laser-ablatable mask layer on the surface are known in the art. A relief image can be produced in such elements without the use of a digital negative image or other imaged element or masking device. Films with a laser-ablatable mask layer can be formed by first imagewise exposing the film with laser radiation (generally an infrared radiation laser under computer control) to selectively remove the mask layer in the exposed areas. The masking film is then placed in contact with a radiation-sensitive element and subjected to overall exposure with actinic radiation (for example, UV radiation) to cure the radiation sensitive element in the unmasked areas and thus form a negative image of the mask in the element. The film containing the mask layer and the imaged radiation-sensitive element (such as an imaged printing plate precursor) are then subjected to solvent development. The unexposed printing plate areas and the mask layer are completely developed off, and after drying, the resulting imaged element is useful, for example as a flexographic printing plate.
While flexographic printing plates having an integral laser-ablatable mask layer allow direct imagewise exposure using a laser and do not require a separate masking device, the time for imaging is generally too long since the system sensitivity to imaging radiation is low. Various attempts have been made in the industry to overcome this problem by increasing the infrared sensitivity of the mask layer. However, obtaining higher sensitivity has been a challenge due to the widely varying quality criteria that must be simultaneously satisfied. In addition, this approach requires the use of high-powered laser-equipped imaging apparatus that is especially configured for imaging flexographic articles. Because of the need for varying the thickness of flexographic plates depending upon the specific intended uses, more than one imaging apparatus may be required for the integral-mask approach.
An important advance in the art of making and using masking films is described in U.S. Patent Application Publication 2005/0227182 (Ali et al., hereinafter cited as US '182). The described method provides a mask image in significantly less time due to greater imaging sensitivity. Problem to be Solved
Although the method of relief image formation, as described in US '182 provides a mask image in significantly less time, it has been observed that when UV exposure is done through the carrier sheet of the mask film, the resulting shoulder angle is lower than desirable. This results in a higher level of halation of the printed image. The higher level of halation is particularly noticeable at higher impression pressure during printing.
For printing, surface quality and properties of a flexographic printing plate are important attributes. In practice, prolonged exposure time is often necessary in order to hold or fully cure the smaller features such as the high-light dots, for example, 1% to 5% dots (where percent refers to the amount of paper covered with print ink) of high quality print images. However, the over-exposure fills in the reverse lines or shadow areas. Thus, over-exposure results in image quality degradation.
The term “exposure latitude” describes the degree to which a photosensitive element can be over-exposed with only negligible image quality degradation. Exposure latitude can be further defined as the ability to simultaneously image low light throughput features, for example, 1-2% dots, and high light throughput features, for example, 4-mil reverse lines, onto a flexographic plate. Photosensitive flexographic printing plates with larger exposure latitude are desirable as they are more tolerant to the actual exposure time used during front image-wise exposure and are thus easier to use.
Halation in flexographic printing is a well known. U.S. Pat. No. 6,864,039 (Cheng Lap Kin et al.) describes halation caused by scattering of the UV light within non-imaged areas of the photopolymerizable medium. As nearly all heterogeneous photocrosslinkable compositions exhibit some degree of light scattering, prolonged image-wise exposure leads to a high level of background scattered actinic radiation, which is often sufficient to cause cross-linking or curing of polymer in regions not exposed to imagewise radiation. The overall effect of such unwanted cross-linking is the filling-in of fine negative that is “halos”, around solid image areas. Halos lead to degradation in the print quality of flexographic printing plates and are linked to dot-gain that is the formation of a larger image dot size than intended. This patent discloses the use of photobleachable compounds in the photopolymer composition to increase resistance to scattered light.
U.S. Pat. No. 5,496,685 (Farber et al.) also describes halation caused by excessive scattering or irregular reflection of light from the support of the printing element, resulting in shallow relief. It also teaches the use of an actinic radiation absorber to improve exposure latitude.
EP 0 504 824A (Swatton et al.) describes the use of antihalation agents in the support of the photopolymer. The antihalation agents are actinic radiation absorbers.
Another cause of halation is the presence of low-angle-of incidence radiation during exposure that can enter the photopolymer below the mask at the edges of exposed areas, reducing the shoulder angles. As the average shoulder angle decreases below 50°, the loss in the relief sharpness becomes increasingly noticeable and as the average shoulder angle decreases below 40° there is a considerable loss in print stability and sharpness. A collimated light source may reduce the halation by reducing the level of lower angle incident light. However, a collimated light source is more expensive to use than commonly used point light or bank light source.
Adding a low refractive index antihalation layer to photographic silver halide films to control unwanted incident or scattered light is described in U.S. Pat. No. 2,481,770 (Nadeau). But the use of such layers in masking films to provide flexographic printing plates is unknown. There is a need to solve the problem caused by lower angle incident radiation in the preparation of relief images in imageable elements such as flexographic printing plate precursors so the relief image predominantly has shoulder angles of at least 50°. There is also a need to improve exposure latitude so that small dots can be retained on a plate without degrading the shadow images and reverse lines.